"Each man individually to his task and his load"—Numbers 4:19.
The kohanim (priests) and Levites each have their unique tasks in the Holy Temple. As such, Levites are forbidden to discharge any of the duties that are the domain of the kohanim, and vice versa.
Indeed, it is also forbidden for a priest or Levite to get involved in a task that was assigned to a fellow priest or Levite. The Talmud relates that Rabbi Joshua ben Chananiah once wished to aid his fellow Levite Rabbi Yochanan ben Gurgoda. "Desist from this," Rabbi Yochanan told him, "for you are already liable to pay with your life. For I am of the gatemen and you are of the singers..."
